ACCUMULATOR HAVING OPERATING FLUID VOLUME INDEPENDENT OF EXTERNAL HYDROSTATIC PRESSURE
A method and device for maintaining pressure in an accumulator for subsea wellbore operations is disclosed.
Latest SCHLUMBERGER TECHNOLOGY CORPORATION Patents:
Not applicable.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENTNot applicable.
BACKGROUNDAccumulators are devices that provide a reserve of hydraulic fluid under pressure. Accumulators are used in, for example, hydraulically-operated systems where hydraulic fluid under pressure operates a piece of equipment or a device. The hydraulic fluid may be pressurized by a pump that maintains the high pressure required.
If the piece of equipment or the device is located a considerable distance from the pump, for example, a significant pressure drop can occur in the hydraulic conduit or pipe which is conveying the fluid from the pump to operate the device. Therefore, the flow may be such that the pressure level at the device is below the pressure required to operate the device. Consequently, operation may be delayed until such a time as the pressure can build up with the fluid being pumped through the hydraulic line. This result occurs, for example, with devices located in a body of water at great depth, such as with a subsea test tree (“SSTT”) and blowout preventer (“BOP”) equipment, which is used to shut off a wellbore to secure an oil or gas well from accidental discharges to the environment. Thus, accumulators may be used to provide a reserve source of pressurized hydraulic fluid for such types of equipment.
In addition, if the pump is not operating, or if no pump is used, accumulators can be used to provide the source of pressurized hydraulic fluid to enable the operation of the piece of equipment or device.
Accumulators conventionally include a compressible fluid, e.g., gas such as nitrogen, helium, air, etc., on one side of a separating mechanism in a pressure resistant container, and a substantially incompressible fluid (e.g., hydraulic oil) on the other side of the separating mechanism. When the hydraulic fluid is released from the accumulator and the system pressure drops below the pressure on the gas side of the separating mechanism, the separating mechanism will move in the direction of the hydraulic fluid side of the separating mechanism, displacing the stored hydraulic fluid into the piece of equipment or the device as required.
When a conventional accumulator is exposed to external hydrostatic pressure, such as encountered in subsea operations, the available volume of hydraulic fluid that can be discharged from the accumulator is decreased because the hydrostatic pressure must first be overcome in order to displace the hydraulic fluid from the accumulator. Once a conventional accumulator begins to displace fluid under such conditions, the pressure of the incompressible fluid decreases and eventually cannot overcome the hydrostatic pressure, thus causing the remaining fluid in the conventional accumulator to become essentially unusable. One technique known in the art for using accumulators exposed to hydrostatic pressure is to compensate for the expected hydrostatic pressure by increasing the gas charge pressure on the gas side of the separator mechanism to compensate for the hydrostatic pressure. In conventional accumulators, the amount of gas pressure (called “precharge”) must usually be selected for the operating depth in a body of water in order to optimize the available hydraulic fluid volume. In a deep subsea well, for example, the required gas precharge pressure may be higher than the hydraulic fluid pressure, rendering the accumulator useless when testing the hydraulic circuit from the surface. A conventional accumulator has the further shortcoming that it cannot be used at different depths; it must be used at the depth for which it is configured or the accumulator may still have a substantial amount of unusable hydraulic fluid.
Pressure-balanced accumulators have been proposed to overcome the above-described shortcomings of a conventional accumulator. One type of pressure-balanced accumulator is disclosed for example, in U.S. Pat. No. 6,202,753 to Baugh. Other examples of pressure balanced accumulators are disclosed, for example, in U.S. Pat. No. 7,628,207 issued to Leonardi et al. and assigned to the assignee of the present invention.
There continues to be a need for improved pressure balanced accumulators.
SUMMARYOne aspect of the invention is an accumulator for subsea wellbore operations including a generally cylindrical housing having a first and second longitudinal end each having a fluid port. The housing is divided into two sections by a bulkhead. A first piston is disposed on one side of the bulkhead and a second piston is disposed on the other side of the bulkhead. A connecting rod disposed between piston and defines an atmospheric pressure or vacuum chamber in a longitudinal end in contact with the second piston. The first piston defines an hydraulic fluid chamber and a gas charge pressure chamber between it and the bulkhead. The second piston defines a hydrostatic pressure chamber between it and the second longitudinal end of the housing. The first and second pistons having substantially equal cross sectional areas on both sides thereof such that a pressure of fluid in the hydraulic fluid pressure chamber is substantially equal to a pressure of gas in the gas charge pressure chamber and a hydrostatic pressure applied to the hydrostatic pressure chamber.
Other aspects and advantages of the invention will be apparent from the description and claims which follow.
An example accumulator according to the invention is shown schematically in
One end of the connecting rod 34 is in contact with a first piston 40. In
In the present example, annular supports 52 may be disposed at selected locations within the gas charge chamber 50 to support the housing 30 and to facilitate longitudinal movement of the connecting rod 34. The annular supports 52 may include passages 54 so that the gas in the pressure charge chamber 50 may move freely therethrough when the accumulator 10 is operated
The first 40 and second 38 pistons preferably each have the same cross-sectional area on the faces thereof exposed, respectively to the hydraulic fluid chamber 42 and the hydrostatic chamber 44. The cross-sectional area of the connecting rod 34 occupies some of the cross sectional area of the interior of the housing 30. Thus, the pressure exerted on the hydraulic fluid in the hydraulic fluid chamber 42 is at a lower pressure than the gas charge pressure, such lower pressure being the product of the gas charge pressure and the ratio of cross sectional area the annular cross sectional area of the gas charge chamber 50 and hydraulic fluid chamber side of the first piston 40. The diameter of the connecting rod 34 may be selected to provide a selected hydraulic fluid pressure given a selected gas charge pressure. For example, the ratio of cross sectional areas on the gas charge pressure chamber and the hydraulic fluid chamber side of the first piston 40 thereof may be ⅗. Thus, a gas charge pressure of 5000 pounds per square inch will provide an hydraulic fluid pressure of 3000 pounds per square inch when there is no hydrostatic pressure applied to the second piston 38.
The second piston 38 has the same cross sectional area as the first piston 40 on the face thereof exposed to the hydrostatic chamber 44, and on its other side is exposed to the atmospheric chamber 36, 36A, 37 defined by the connecting rod 34, which has the same annular cross sectional area as the gas charge pressure chamber 50 as a result of some of the cross section being occupied by the connecting rod 34 (which may have a substantially constant diameter).
As the accumulator 10 is moved deeper into the water (22 in
A possible advantage of having the various chambers in the accumulator 10 arranged as shown in and explained with reference to
While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims
Claims
1. An accumulator for subsea wellbore operations having hydraulic charge pressure maintained at a substantially constant pressure above hydrostatic pressure at any depth in a body of water, the accumulator comprising:
- a generally cylindrical housing having a first longitudinal end and a second longitudinal end, each longitudinal and having a port therein, the housing divided into two sections by a bulkhead;
- a first piston disposed in the housing on one side of the bulkhead;
- a second piston disposed in the housing on the other side of the bulkhead;
- a connecting rod disposed between the first and second pistons, the connecting rod having an atmospheric pressure or vacuum chamber defined in a longitudinal end thereof in contact with the second piston; and
- wherein the first piston defines an hydraulic fluid chamber between the first piston and the first longitudinal end of the housing and a gas charge pressure chamber disposed between the bulkhead and the first piston, and wherein the second piston defines a hydrostatic pressure chamber between the second piston and the second longitudinal end of the housing, the first and second pistons having substantially equal cross sectional areas on both sides thereof such that a pressure of fluid in the hydraulic fluid pressure chamber is substantially equal to a pressure of gas in the gas charge pressure chamber and a hydrostatic pressure applied to the hydrostatic pressure chamber.
2. The accumulator of claim 1 further comprising annular supports disposed in the gas charge pressure chamber, the annular supports having openings generally in a center thereof to enable passage of the connecting rod and openings to enable flow of gas in the gas charge pressure chamber.
3. The accumulator of claim 1 wherein the accumulator is disposed in a riser coupled to a subsea test tree, and the port in the first longitudinal end of the housing is coupled to the subsea test tree to provide hydraulic pressure to operate components thereof.
4. The accumulator of claim 3 wherein the port in the second longitudinal end of the housing is exposed to fluid in the riser.
5. The accumulator of claim 1 wherein a ratio of cross sectional area of the first piston exposed to the hydraulic fluid chamber and exposed to the gas charge pressure chamber is selected such that a selected gas charge pressure results in a selected hydraulic fluid pressure.
6. The accumulator of claim 1 wherein an initial pressure of the gas charge is about 5,000 pounds per square inch.
7. The accumulator of claim 6 wherein an initial pressure of the hydraulic fluid is about 3,000 pounds per square inch plus the hydrostatic pressure.
8. A method for maintaining pressure in an accumulator for subsea wellbore operations having hydraulic fluid pressure maintained at a substantially constant pressure above hydrostatic pressure at any depth in a body of water, the method comprising:
- communicating pressure of a pressurized gas in a container to a body of hydraulic fluid and to a chamber having at least one of atmospheric pressure and vacuum therein;
- communicating hydrostatic pressure to the body of hydraulic fluid such that a pressure of the hydraulic fluid is substantially always equal to a sum of the pressure in the container and the hydrostatic pressure.
9. The method of claim 8 further comprising coupling the body of hydraulic fluid to a control in a subsea test tree.
10. The method of claim 8 further comprising discharging the hydraulic fluid to operate the control.
11. The method of claim 8 wherein the communicating hydrostatic pressure is performed by a first and a second piston each disposed at an opposed longitudinal end of a connecting rod, the connecting rod passing through the container, wherein the hydrostatic pressure exerted on the first piston is communicated through the connecting rod to the second piston in contact with the hydraulic fluid.
12. The method of claim 8 wherein the first piston separates compartments in an accumulator housing having the hydrostatic pressure in a first compartment and the at least one of atmospheric pressure and vacuum in a second compartment.
13. The method of claim 8 wherein an initial pressure of the pressurized gas is about 5,000 pounds per square inch.
14. The method of claim 8 wherein an initial pressure of the hydraulic fluid is about 3,000 pounds per square inch plus the hydrostatic pressure.
15. An accumulator for subsea wellbore operations having hydraulic charge pressure maintained at a substantially constant pressure above hydrostatic pressure at any depth in a body of water, the accumulator comprising:
- a generally cylindrical housing having a first longitudinal end and a second longitudinal end, each longitudinal and having a port therein, the housing divided into two sections by a bulkhead;
- a first piston disposed in the housing on one side of the bulkhead;
- a second piston disposed in the housing on the other side of the bulkhead;
- a connecting rod disposed between the first and second pistons, the connecting rod having an atmospheric pressure or vacuum chamber defined in a longitudinal end thereof in contact with the second piston;
- wherein the first piston defines an hydraulic fluid chamber between the first piston and the first longitudinal end of the housing and a gas charge pressure chamber disposed between the bulkhead and the first piston, and wherein the second piston defines a hydrostatic pressure chamber between the second piston and the second longitudinal end of the housing, the first and second pistons having substantially equal cross sectional areas on both sides thereof such that a pressure of fluid in the hydraulic fluid pressure chamber is substantially equal to a pressure of gas in the gas charge pressure chamber and a hydrostatic pressure applied to the hydrostatic pressure chamber; and
- wherein the accumulator is disposed in a riser coupled to a subsea test tree, and the port in the first longitudinal end of the housing is coupled to the subsea test tree to provide hydraulic pressure to operate components thereof.
16. The accumulator of claim 15 further comprising annular supports disposed in the gas charge pressure chamber, the annular supports having openings generally in a center thereof to enable passage of the connecting rod and openings to enable flow of gas in the gas charge pressure chamber.
17. The accumulator of claim 15 wherein the port in the second longitudinal end of the housing is exposed to fluid in the riser.
18. The accumulator of claim 15 wherein a ratio of cross sectional area of the first piston exposed to the hydraulic fluid chamber and exposed to the gas charge pressure chamber is selected such that a selected gas charge pressure results in a selected hydraulic fluid pressure.
19. The accumulator of claim 15 wherein an initial pressure of the gas charge is about 5,000 pounds per square inch.
20. The accumulator of claim 15 wherein an initial pressure of the hydraulic fluid is about 3,000 pounds per square inch plus the hydrostatic pressure.
Type: Application
Filed: Sep 13, 2011
Publication Date: Mar 14, 2013
Applicant: SCHLUMBERGER TECHNOLOGY CORPORATION (Sugar Land, TX)
Inventor: Quangen DU (Fresno, TX)
Application Number: 13/231,740
International Classification: E21B 23/00 (20060101); E21B 41/00 (20060101);